The present invention relates generally to nondestructive testing of composite materials or panels, particularly wood based materials, such as plywood, oriented strand board, wafer board, particle board, and the like, to determine the strength and stiffness of such panels.
The use and acceptance of composite materials and panels for various applications, such as, building constructions, continues to increase in the market place. As a result, it is becoming increasingly desirable to monitor the strength and stiffness of the panels being produced. This is so because the strength and stiffness of composite materials varies greatly due to the composite nature of the products and the difficulty in achieving uniform strength in the bonding materials used to join the composites together. Moreover, variations in feedstocks and other factors make manufacture of uniformly strong and elastic structures from composite elements difficult and costly.
Nondestructive inspection and testing of materials of all sorts is known. Many of the known methods for performing certain standards tests are manual or static methods. For example, to conduct a concentrated load test, it is known to build a frame with beams simulating joists in a building construction. The beams are spaced apart depending upon the end use and span rating of the panel to be tested. A hydraulically-actuated load is applied to the stationary panel at a specified distance from a non-secured edge and the deflection of the panel is measured by placing a dial-micrometer under the panel at a position opposite the load and reading the deflection on the micrometer scale.
U.S. Pat. No. 4,708,020 to Lau et al., which is incorporated herein by reference, relates to another form of nondestructive inspection and testing of composite panels to determine the strength and stiffness of the panels. More particularly, Lau et al. provide an apparatus and process for correlating end-use strength and stiffness values when the testing is carried out on hot panels. The panels may be tested at one temperature, approaching the press temperature, and the strength and stiffness determined for the end products at another temperature, generally ambient or end-use temperature. Lau et al. also provide a testing machine suitable for in-line testing for determining the strength and stiffness of panel products having different thicknesses. The testing machine of Lau et al. also enables panels to be graded so that rejects can be identified and panels can be separated into grade groups representing different strength and stiffness ranges.
The continuous panel tester of Lau et al. imposes a double reverse bend or xe2x80x9cSxe2x80x9d shaped configuration on the panels as they pass through the conveyor at line speed. The device of Lau et al. is configured and operated such that either the deflection of each panel may be measured for a specific load, or the load is measured for a particular deflection of each panel.
As set forth in Lau et al., there is provided a first in-feed roll and a last out-feed roll to direct each panel to be tested into and out of the overall continuous panel tester and grader. As also described in Lau et al., a plurality of photo switches along the conveyor line have the function of informing the microprocessor when a panel is in the tester. The photo switches of Lau et al. determine when one panel ends and a second panel commences to pass through the tester so as to ensure that readings from the load cells and temperature sensor represent strength and stiffness figures for one panel. Another feature of Lau et al. is the ability of the panel grader to test panels having different thicknesses by merely selecting the required nominal panel thickness. The microprocessor is programmed to control the necessary equipment to position the rolls of the apparatus to process the panels of the selected nominal thickness. Based on the selected nominal thickness which is inputted to the microprocessor, the microprocessor utilizes information received from the load cells and temperature sensor to calculate the hot strength and stiffness values for each panel and then the microprocessor uses a preprogrammed algorithm to determine the ambient or cold end-use strength and stiffness value for each of the tested panels. Lau et al. do provide that it may be desirable to use a thickness measuring sensor such as a laser sensor or an ultrasonic sensor, which is placed near the in-feed rolls of the panel tester, to obtain a more actual thickness measurement of each panel, as compared to using the selected nominal thickness for each panel, thereby providing for a more accurate calculation of the strength and stiffness properties for each panel.
Despite the increased use of composite materials for all sorts of building constructions and other uses, and the general desire to test the composite materials for strength and stiffness, a need still exists for an improved panel tester and grader which is efficient and economical in its manufacture and use and which also provides improved accuracy in terms of measuring and grading panel like products according to desired strength and stiffness values.
As can be appreciated by those skilled in the art, the many known manual methods for performing certain standard tests for panels or the like are generally labor intensive, slow processing, somewhat costly procedures that can readily lead to error or operator mistakes when trying to determine the strength and stiffness values for panels. Moreover, the known static testing machines do not allow a panel to continually move along the production line during testing, thereby limiting the usefulness of such testing equipment.
Although Lau et al. describe an automatic, continuous panel tester and grader which is in many aspects an improvement over the known manual or static methods, the device of Lau et al. also exhibits several problems. One problem with Lau et al. concerns the bending forces that are applied to the panels as they are fed to and passed out of the panel tester. Although Lau et al. recognize that no significant forces should be applied to the panels that would distort the loading forces of the panels in the xe2x80x9cSxe2x80x9d shaped path, it has actually been determined according to the present invention that the first in-feed roller (40) and the last out-feed roller (70) of Lau et al. (see FIG. 2 thereof) do in fact apply undesirable bending forces or moments to the panels as they travel thereover, thereby resulting in significantly less than accurate strength and stiffness values for the tested panels. It has been determined according to the present invention that if the panels are subjected to a bending force outside the critical load zone or path, the deflection for a specific load or the load applied for a particular deflection may be greater than or less than what the actual deflection or load would be absent the undesirable bending force, depending on the direction the panels are caused to bend outside the load zone.
Another problem with Lau et al. concerns the location of the photo switches (1)-(4) (see FIG. 1 thereof) which communicate with the microprocessor (22) so that the microprocessor knows when to begin and when to end taking and recording loading and temperature readings for a specific panel traveling through the panel tester. Lau et al. disclose that a composite panel (10) moves in an xe2x80x9cSxe2x80x9d shaped path through the tester. The first deflector roll (14) is positioned midway between a first pair of spaced positioning rolls (13) each of which cooperates with its respective reaction roll (50) to clamp the panel (10) therebetween, all of which function to bend the panel in a first direction in the first curved portion of the xe2x80x9cSxe2x80x9d shaped path. The second deflection roll (16) is positioned substantially midway between a second pair of positioning rolls (13) each of which cooperates with its respective reaction roll (60) to clamp the panel (10) therebetween, all of which function to bend the panel in a second direction opposite to the first direction in the second curved portion of the xe2x80x9cSxe2x80x9d shaped path, i.e., in a reverse curvature to that forced by the first deflection roll (14). According to Lau et al., when the photo switches indicate that a panel is in the tester, readings from the load cells (18) and temperature sensor (24) are taken at predetermined intervals and the microprocessor uses these readings to calculate a strength and stiffness value for each panel tested. As shown and described in Lau et al., the photo switches are placed along the processing line with no particular regard as to how their placement may affect the calculated strength and stiffness values. In other words, what Lau et al. fail to recognize, and what has been determined according to the present invention, is that the location of the photo switches or sensors relative to the load zone of the xe2x80x9cSxe2x80x9d shaped path is important in terms of the overall calculated strength and stiffness value for each panel tested.
According to the present invention, it has been determined that in order to compute more accurate strength and stiffness values for the panels, each panel should be subjected to bending forces in the first and second curved portions of the xe2x80x9cSxe2x80x9d shaped path or load zone between the pairs of opposed positioning and reaction rolls adjacent to the respective deflector rolls. Any forces or adverse bending moments applied to the panels outside the load zone which causes the panels to bend in an undesirable manner, will result in less than accurate strength and stiffness values. Accordingly, since the panels should only be subjected to the appropriate bending forces within the load zone, and since the microprocessor calculates a strength and stiffness value for each panel traveling through the panel tester, it is desirable for the microprocessor to take and record the desired measurement readings only when each panel is in or substantially in the load zone of the xe2x80x9cSxe2x80x9d shaped path as defined between the pairs of opposed positioning and reaction rolls. Locating the photo switches as illustrated in Lau et al. results in the microprocessor taking and recording the load and temperature readings for the panels when the panels are not properly in the defined load zone of the xe2x80x9cSxe2x80x9d shaped path, thereby undesirably skewing the calculated strength and stiffness values for the panels.
Yet another problem with Lau et al. is that the panel tester and grader does not provide a mechanism to measure the thickness of each panel tested with a high degree of accuracy. As explained in Lau et al., a thickness value for the panels is needed in order to calculate the strength and stiffness values for the panels. In the preferred embodiment of Lau et al., a nominal thickness value for a set of panels (see, e.g., TABLES I and II therein and the description thereof) is simply inputted into the microprocessor, so that the appropriate calculations can be made. As noted, Lau et al. do teach that if a more accurate calculation of strength and stiffness is desired, a thickness sensor such as a laser sensor or an ultrasonic sensor may be used to measure the actual thickness instead of using the nominal thickness of each panel. Even so, what Lau et al. fail to recognize, and what has been determined according to the present invention, is that the thickness of each panel is a very significant parameter in determining the most precise measure of the strength and stiffness value for each tested panel. For example, a laser sensor will only measure the thickness of a panel at the specific location where the laser contacts the panel. As can be appreciated by those skilled in the art, panels of the type described herein can have varying thicknesses over the length and width of each panel. A single laser sensor cannot take into account the varying thicknesses throughout the panels. As a result, the averaged thickness measurement obtained by a laser sensor may not be a true representative measurement of the overall thickness of the particular panel. It is possible that multiple laser sensors could be used to improve the accuracy of the averaged thickness measurement for each panel, but multiple sensors would add undesirable cost and complexity to the overall panel tester, thereby resulting in a less than optimum machine. Likewise, an ultrasonic sensor will simply not provide accurate thickness measurements. As can be appreciated by those skilled in the art, panels of the type described herein have a tendency to vibrate as they are processed along the continuously operating panel tester and grader. Such vibrations in the panels will undoubtedly adversely affect the readings taken by an ultrasonic thickness tester. Thus, according to the present invention, it has been determined that in order to obtain a more accurate calculated strength and stiffness value for each panel, a new and improved thickness measuring device is required.
In sum, what is needed is a panel tester and grader that improves on the apparatus and method described in Lau et al., thereby providing a more accurate account of the strength and stiffness properties of each panel tested.
The present invention provides a panel tester for testing individual panels delivered to the panel tester in a stream. The panel tester comprises a main frame including a plurality of rollers and defining a substantially S-shaped testing path. A substantially linear bypass path extends below the testing path. A bypass assembly is coupled to the main frame and is operable to move the main frame between a testing position in which panels are received by the testing path, and a bypass position in which panels are received by the bypass path.
According to another aspect of the present invention, the panel tester includes at least one sensor operatively coupled to a roller to measure a load applied to the roller. As the panel moves along the testing path, the load changes. A control system is provided and communicates with the sensor to monitor the load. As the panel moves along the testing path and the load changes, the control system detects the changes in the load and selectively records the load in response to the detected load changes. Specifically, the testing path includes substantially linear portions, and substantially curved portions, and as the panel moves from a linear portion to a curved portion of the testing path, the load increases. As a result of the load increasing, the control system begins recording the load. Similarly, when the panel moves from a curved portion to a linear portion of the testing path, the load decreases, and in response to the load decreasing, the control system ceases recording the load.
In a preferred embodiment, the bypass assembly includes a lifter assembly that lifts the main frame to the bypass position. When the main frame is in the bypass position, the lifter assembly and the bypass frame support the main frame. The main frame includes a conveyor assembly that substantially defines the bypass path and the bypass path extends below the testing path.
The present invention also provides a method for testing individual panels having a leading edge and delivered to a panel tester in a stream. The method includes guiding an individual panel through the panel tester along a substantially S-shaped testing path that is defined by a plurality of rollers. A load cell providing a load output is coupled to a deflector roller to measure a load applied to the panel by the deflector roller. The load output is monitored as the panels are guided through the panel tester. The leading edge of the panel is fed along the S-shaped path in a generally linear direction and the panel is bent by diverting the leading edge of the panel into a nip defined between two adjacent rollers. As the panel is bent, the panel engages the deflector roller and the load output increases. When the load output increases, the load output is recorded by a control system, and when the panel disengages the deflector roller and the load output decreases, the control system ceases recording the load.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features.